enhanced the error message of pick function.

When pick function fail return a better error message like
the other stdlib functions, indicating that the error
is on function pick.

This would help people that see the error to identity it is
related to a incorrect use of stdlib function pick, instead of having
to grep all puppet libraries and manifests source for the old message.

I had also changed the spec test.

pick function change spec  as suggested GH-179

Fix the spec test to use expect {}.to instead of lambda {}.should
as explained by Adrienthebo.

"Using expect { }.to is preferred over lambda { }.should.
 In addition it's best practice to do a string match against the
error message to ensure that we're catching the right error,
instead of any error of the right type."

Also fixed a typo on the error message, it was missing one space.

pick function stylish fix as suggested on GH179
Merge pull request #179 from lmello/function_pick_error_msg

enhanced the error message of pick function.